from collections.abc import Mapping


class ValueImmutableDict(Mapping):
    _EMPTY_VALUE = object()

    def __init__(self, somedict):
        self._dict = dict(somedict)  # make a copy
        self._hash = None

    def __setitem__(self, key, value):
        cur_value = self._dict.get(key, self._EMPTY_VALUE)
        if cur_value is not self._EMPTY_VALUE and cur_value != value:
            raise KeyError(f"can't change value of key: k={key},v={cur_value},new_v={value}")

        self._dict[key] = value

    def __getitem__(self, key):
        return self._dict[key]

    def __len__(self):
        return len(self._dict)

    def __iter__(self):
        return iter(self._dict)

    def __hash__(self):
        if self._hash is None:
            self._hash = hash(frozenset(self._dict.items()))
        return self._hash

    def __eq__(self, other):
        return self._dict == other._dict